1. Field of the Invention
The present invention relates generally to pressure holding devices or vessels, and, more specifically, to downhole pressure gauges or pressure transducers commonly used in the oil industry. More particularly, this invention relates to a seal for use on such pressure gauges or pressure transducers.
2. Background
Downhole pressure gauges are beneficial for use in oil production. One of the parameters used in determining the amount of oil in a reservoir is pressure. The pressure at the bottom of a well bore provides reliable data because such pressure is the pressure at the face of the oil formation. Downhole pressure is important to production because it allows the prediction of production rates, oil reserves, and the remaining volume in the formation. It is possible to determine the volume of oil in the formation if the amount of oil removed from the formation is known and the drop in pressure can be determined at the formation face.
The pressure at the formation face is also beneficial for sizing and running the well pumps so that such pumps are not allowed to run dry. Additionally, if water is injected into a well at the surface, the downhole pressure must be monitored. If downhole pressure becomes too high, a risk of fracturing and damaging the hole arises. Therefore, a continuous monitoring of the downhole pressure is necessary to produce at the correct rate.
Monitoring of downhole pressure is accomplished by the use of a pressure gauge. The gauge may be a permanent gauge, such as a tubing installed gauge, or a temporary gauge maintained on a wireline or electric line. In the case of a tubing installed gauge, the pressure is routed to the gauge from inside the tubing. In some designs, this creates a high internal pressure in the adapter that attaches the pressure transducer to the tubing.
It is desirable to minimize the outside diameter of the gauge to allow the gauge to be used in smaller tubing or a smaller annulus. The minimization of dimensions leads to the use of ultra-high strength materials with thin walls for pressure gauge design. Pressure gauges having thin walls are more apt to fail at the seal due to the outward expansion of the thin walls caused by the tremendous internal pressures. This effect is not diminished by the use of ultra-high strength materials since the expansion is not dependent upon the strength of the material. The expansion is dependent upon the geometry, temperature, and the modulus of elasticity, which are not necessarily improved by using a stronger material. However, stronger materials do delay the onset of plastic deformation, which is where expansion rapidly increases.
Conventional seals typically utilize elastomeric materials which have a short service life when exposed to corrosive fluids and high temperatures. Corrosive fluids and high temperatures are conditions often encountered in oil and gas wells. Since it is expensive to pull a permanent downhole gauge, it is beneficial to have a gauge with a long service life so that the gauge can remain downhole for a long period of time. One solution to this problem is to use a metal seal such as a weld or a metal to metal contact seal.
Many permanent gauges have two or more housing components joined by welds. However, if the gauge must be attached to the tubing at the well site, welding is not an option since it is not permitted at the well site for safety reasons. Therefore, the seal between the gauge and the mandrel can not be welded joint.
Though temporary gauges do not have long downhole residency requirements, they often have thin walls in order to allow room for the necessary internal components. These gauges normally have a sealed pressure housing and are exposed to corrosive fluids and high temperatures. An inert gas atmosphere, such as helium or argon or a mixture of the two, can be used to protect the internal electronics from oxidizing at high temperatures.
A metal seal is desirable because it will not deteriorate over time like an elastomeric seal when exposed to high temperature corrosive fluids. This is very desirable when an inert gas is used because the gas would escape from the pressure housing when the elastomeric seals were replaced. It is desirable to use a secondary o-ring seal in case the metal seal would be damaged or worn due to maintenance procedures, which should be greatly reduced or eliminated with the use of an inert gas and metal seals.
When used as a high pressure seal, conventional metal seals fail to overcome the limitations discussed above as they may bell out due to excessive torque and thus lose their seal.
Consequently, it is an object of this invention to create a seal for pressure holding devices or vessels, and especially downhole pressure transducers, that does not need to be maintained, that does not require welding, that does not utilize an elastomeric seal as the primary seal.
It is a further object of this invention that the seal require less makeup torque than conventional metal seals to form a seal for high pressure applications.